Storage Area Networks: Designing and Implementing a Mass Storage System

3.11 FC-3: Common Services

The FC-3 level, located at the center of the functional levels, concerns itself with functions spanning multiple N_Ports. The FC-3 level is the single point in the architecture through which all traffic must flow in both directions. The FC-3 level will contain services common (available) to all ports on a node.

A node may have several ports. A node may also have several ULPs and FC-4 level mappings. However, there is only one FC-3 Common Services level per node. The FC-3 level can manage a set of tables holding the login information for other active ports. Each port on the FC-3 level knows which ports are busy and which exchanges they are busy with.

Figure 3-9 on page 57 shows where the FC-3 level fits into the overall scheme of all the Fibre Channel levels.

Currently there are three functions defined within the FC-3 level standard:

• Striping ”   Used to achieve higher bandwidth. Striping allows multiple links simultaneously and transmits a single information unit across multiple links employing multiple N_Ports in parallel.

• Hunt Groups ”   Are a group of N_Ports associated with a single node. They permit any N_Port on the node to receive information containing the correct alias identifier.

• Multicast ”   This can be compared to a broadcast message. It allows a single information unit to be transmitted to multiple N_Ports on a node.

The FC-3 level knows nothing about the topology of Fibre Channel or the physical signaling at the lower levels. This is handled by FC-1 and FC-2 levels. FC-3 understands if there are multiple ports attached to a node and if they may participate in multiport operations like multicasting.

Knowing which ports are busy allows the FC-3 level to route exchanges between two N*_Ports and FC-4s.